The invention relates to a high frequency heating apparatus or a microwave oven with a function for properly heating food stuffs and, more particularly, a high frequency heating apparatus or a microwave oven in which infrared rays radiated from food stuffs within a heating chamber is detected so as to control an optimum time of high frequency heating.
In a conventional high frequency heating apparatus such as a microwave oven, a microwave radiation time is set by using a timer switch in accordance with an instruction described in a cookbook or depending on an experience of a user. However, it is very difficult to always set a proper time. For this, a user frequently sets an improper cooking time so that victuals are dehydrated excessively, resulting in impared taste. When a cooking time set is insufficient, the victuals must be reheated.
With a view of overcoming those problems, there have been many proposals in which infrared rays radiated from food stuffs being cooked are detected and, when the temperature of the food stuffs reaches a proper value, the heating is automatically stopped or is continued for a given time while keeping the temperature of the food stuffs at a proper value. For example, U.S. Pat. No. 4,049,938 to Akihiko Ueno issued Sept. 20, 1977 discloses a microwave oven including infrared ray detecting means for detecting infrared rays from at least two detection points within a heating chamber of the microwave oven. In spite of its complicated electrical circuit construction and expensive parts, the apparatus of this type has poor accuracy in temperature detection as well as in temperature control.
The amount of infrared rays radiated from food stuffs depends on the surface area of the food stuffs as well as the temperature of the same. The amount of infrared rays radiated from the food stuffs increases proportional to the surface area of the food stuffs. If the temperature detection is conducted without taking a proper countermeasure for such surface area dependency, the temperature detected changes depending on the surface area of the food stuffs, thus resulting in poor accuracy in the temperature detection. To solve this problem of the poor accuracy in temperature detection, an infrared detector has been experimented by the inventors which is provided with a cylindrical hood in order to make incident or incoming infrared ray energy constant. However, this attempt has failed to attain an expected effect because of the presence of reflected infrared rays from the hood inner surface. To eliminate the adverse influence of the reflected infrared rays, some approaches have been made such as shortening the diameter of the hood or elongating the length of the hood. When such an approach is employed, however, the incident infrared ray energy decreases so that S/N ratio is deteriorated thus attaining insufficient control.